1. Technical Field
The present invention is directed to an optical assembly or module containing at least two optical devices coupled together, and in particular, the housing for the optical devices in the package.
2. Art Background
In order to accommodate the ever-increasing demand for high-speed, high-volume communication of information, optical communication systems are being developed at a rapid pace. Optical communication systems provide high-speed communications because they provide higher bandwidth (i.e. they are capable of simultaneously transmitting a higher number of signals) than standard electronic communication systems.
Optical communication systems have, at their core, optical fibers for the transmission of optical signals that carry information. These optical fibers interface with a variety of optical modules that introduce signals into and receive signals from the optical fibers. The optical modules perform an array of functions.
Optical modules typically have one or more optical devices and associated electronics therein. Such optical modules are referred to as optoelectronics because of the presence of both optical and electrical components therein.
Packaging for optical modules presents a variety of issues. Packaging issues for optical modules are described in U.S. Pat. No. 5,337,396 to Chen et al. and U.S. Pat. No. 5,596,665 to Kurishama et al. As noted in Kurishama et al., one predominant issue is alignment precision between optical components (e.g. semiconductor lasers, photodiodes, etc.) and optical fibers. In Kurishama et al., the optical component is placed in a sleeve and the sleeve containing the optical component is placed in the housing for the module. After the optical component is placed in the housing, further steps are taken to assemble the module. These steps include forming electrical and optical interconnections between the optical components and other portions of the module (i.e. a ferrule for optical interconnection, an electrical circuit for controlling the laser, etc.). Alignment of the optical component must be maintained during these subsequent assembly steps. Also, the module must be assembled so that alignment is preserved when the module is shipped and handled. Alignment must also be preserved when the module is exposed to operating temperature conditions as well as changes in temperature under operating bias when in service.
The housing for optical components is typically made of metal or ceramic at least in part to provide mechanical strength. Significant cost reduction is possible if such housings are made of plastic. One problem associated with plastic housings is that they shrink or expand in response to respective decreases and increases in temperature. When optical components are packaged into the plastic housing, they are subjected to changes in temperature during steps such as soldering, wirebonding etc. The shrinkage or expansion of the assembly that can occur as a result of the temperature changes associated with the assembly steps can adversely affect the alignment of the optical devices in the optical module. Consequently, housings for optical components that undergo shrinkage or expansion when subjected to a change in temperature, compared to current housings, are sought.
The present invention is directed to a housing adapted to receive an optical device. The housing is then incorporated into an optical assembly or module. A key aspect of an optical assembly is the alignment of the optical components within the assembly. An assembly in which the components are not aligned with acceptable tolerances (typically less than or equal to 5 xcexcm, depending on optical power requirements) will not function properly. As previously noted, a housing that undergoes a change in dimension in response to a change in temperature can adversely affect the alignment of an optical component within an optical assembly and thus render the device a failure. The housing of the present invention is less susceptible to undesired changes in dimension than previous housings.
The housing or module of the present invention has one or more molded polyphenylene sulfide components. The polyphenylene sulfide component is formed by introducing polyphenylene sulfide into a mold at a temperature of about 110xc2x0 C. to about 150xc2x0 C. It is advantageous if the mold temperature is about 120xc2x0 C. to about 150xc2x0 C. Molding the polyphenylene sulfide component at this temperature ensures that the polyphenylene sulfide component is at least fifty percent crystalline. As used herein, percent crystallinity is the percent of the material that exhibits a polycrystalline morphology compared to an amorphous morphology. Once skilled in the art can readily make this determination. For example, using a differential scanning calorimeter (DSC), the experimental heat (xcex94Qf) required to melt the crystals is first determined. The xcex94Qf is then divided by the heat of fusion for PPS (71 J/g). The ratio of xcex94Qf/xcex94Hf times 100 equals percent crystallinity ("khgr").
After molding, the component is then annealed under conditions that increase the portion that is polycrystalline. This is accomplished by annealing at a temperature that is at least above the glass transition temperature (Tg) of the polyphenylene sulfide. The Tg is defined as the temperature of half-vitrification (xc2xd xcex94Cp). Cp is the specific heat (as measured by DSC) and xcex94Cp is the difference between the specific heat of the liquid state (Cp liquid) and the specific heat of the glass state (Cp glass) at the Tg. Alignment and alignment stability requirements determine the amount of crystallinity that is desired for a given polyphenylene sulfide material.
The applicants have observed that the polyphenylene sulfide undergoes both irreversible and reversible dimensional changes in response to changes in temperature. Applicants have discovered that there is a relationship between the crystallinity of the polyphenylene sulfide and the irreversible changes in dimension that are caused by changes in temperature. The process of the present invention forms a polyphenylene sulfide component with increased crystallinity over prior art components. As a result of the increased crystallinity, the polyphenylene sulfide components formed by the process of the present invention undergo smaller irreversible changes in dimension when subjected to temperatures above their Tg (compared to polyphenylene sulfide components not formed by the present process).
As a result of the process of the present invention, the irreversible dimensional changes of the polyphenylene sulfide component in response to a thermal cycle is less than 0.06 percent or less than 0.02 percent when annealed. As used herein, a thermal cycle is one in which the temperature of the polyphenylene sulfide component goes from about room temperature (e.g. 20xc2x0 C.) to a high processing temperature and then back to 20xc2x0 C. In the context of assembling optoelectronic devices, high temperatures are typically in the range of about 150xc2x0 C. to about 200xc2x0 C.